Nanoparticles In Polymer Solutions Research Articles

Phase behavior of colloidal nanoparticles and their enhancement effect on the rheological properties of polymer solutions and gels.

The interaction between nanoparticles and polymers has been of great interest in colloidal theory and novel materials. For example, the properties of polyacrylamide solutions and gels, which are usually used for conformance control and water shut-off in oilfields, can be improved with the addition of nanoparticles. This underlying mechanism and its applicability are investigated in this paper. A strong relationship between the phase behaviors of nanoparticles in polymer solutions and their enhancement effect on the rheology of the nanocomposite polymer solutions and gels was observed. Experiment results showed that the stability of nanoparticles was dependent on several factors, including pH, salinity, and polymer type. At neutral pH conditions, the tendency of the aggregation of nanoparticles was strengthened upon increasing the salinity, polymer concentration, and electronegativity of the polymers. Rheological measurements showed that nanoparticles could improve the viscosity of polymer solutions or the fracture stress of gels only if nanoparticles were aggregated in the corresponding systems. In addition, these rheological parameters significantly increased with increasing salinity and nanoparticle concentration. As a result, the mobility ratio of polymer solutions may be increased several times by the addition of nanoparticles. Referring to the gels, their rupture pressure gradient in the ideal model was also found to increase with nanoparticle concentration. In particular, if the nanoparticle concentration was sufficiently high (reaching 2%), the formed gels would not be destroyed by the injected water, but rather functionally act as a porous medium for permeation.

Temperature-mediated diffusion of nanoparticles in semidilute polymer solutions.

The temperature is often a critical factor affecting the diffusion of nanoparticles in complex physiological media, but its specific effects are still to be fully understood. Here, we constructed a temperature-regulated model of semidilute polymer solution and experimentally investigated the temperature-mediated diffusion of nanoparticles using the particle tracking method. By examining the ensemble-averaged mean square displacements (MSDs), we found that the MSD grows gradually as the temperature increases while the transition time from sublinear to linear stage in MSD decreases. Meanwhile, the temperature-dependent measured diffusivity of the nanoparticles shows an exponential growth. We revealed that these temperature-mediated changes are determined by the composite effect of the macroscale property of polymer solution and the microscale dynamics of polymer chain as well as nanoparticles. Furthermore, the measured non-Gaussian displacement probability distributions were found to exhibit non-Gaussian fat tails, and the tailed distribution is enhanced as the temperature increases. The non-Gaussianity was calculated and found to vary in the same trend with the tailed distribution, suggesting the occurrence of hopping events. This temperature-mediated non-Gaussian feature validates the recent theory of thermally induced activated hopping. Our results highlight the temperature-mediated changes in diffusive transport of nanoparticles in polymer solutions and may provide the possible strategy to improve drug delivery in physiological media.

Combined magnetic and electric field processing of polymer matrix composites for orthogonal control of hierarchical particle arrangements

Properties of particulate-filled polymer matrix composites are highly dependent on the spatial position, orientation and assembly of the particles throughout the matrix. External fields such as electric and magnetic have been individually used to orient, position and assemble micro and nanoparticles in polymer solutions and their resulting material properties were investigated, but the combined effect of using more than one external field on the material properties has not been studied in detail. Applying different configurations of electric and magnetic fields on geometrically and magnetically anisotropic particulates can produce varying microarchitectures with a range of material properties. Experimentally and with simulations, we systematically probe the effect of combined electric and magnetic fields on the microstructure formation of geometrically and magnetically anisotropic barium hexaferrite (BHF) in polydimethylsiloxane (PDMS). The magnetic and dielectric properties resulting from different microstructures are characterized and microstructure-property relationships are analyzed. Our results demonstrate that a variety of microarchitectures can be produced using multi-field processing depending on the nature of the applied external field. For example, the application of an electric field creates macro-chains where the orientation of the BHF stacks inside the macro-chains is random. On the other hand, application of a magnetic field rotates the BHF stacks within the macro-chain in the direction dictated by the magnetic field. In simulations, the dielectrophoretic, magnetic, and viscous forces and torques acting on the particles show that particle anisotropies are central to the ability to control orientation along the orthogonal magnetic and geometric axes, mirroring experimental results. The authors refer to the ability to manipulate particle orientation along orthogonal axes as ‘orthogonal control’. Using this technique, not only are a variety of microstructures possible, but also a range of dielectric and magnetic properties can result. For example, for 1 vol% BHF-PDMS composites, the experimental dielectric permittivity is found to vary from 2.84 to 5.12 and the squareness ratio (remnant magnetization over saturation magnetization) is found to vary from 0.55 to 0.92 (from 0.52 to 0.99 in simulations) depending on the applied external stimuli. The ability to predict and produce a variety of microstructures with a range of properties from a single material set will be particularly beneficial for resin pool based additive manufacturing and 3D printing.

Achieving ultra-high discharge energy density of dielectric polymer nanocomposites by in situ synthesis

Polymer dielectrics are promising dielectric materials for high-energy-density dielectric capacitors owing to their facile processability and high breakdown strength. Currently, the primary method used to prepare nanocomposites involves the combination of polymers and nanoparticles and utilises the high insulation abilities of nanoparticles to limit the migration of carriers to obtain a high breakdown strength and charge–discharge efficiency. However, the dispersion of nanofillers in nanocomposites prepared using traditional methods is unsatisfactory, which not only leads to unsatisfactory energy-storage performance but also limits their applications owing to inconsistent batch-to-batch performance. Herein, nanocomposites with high energy densities are prepared through the in situ synthesis of nanoparticles in polymer solutions. Owing to the uniform dispersion and excellent insulation performance of the nanoparticles, the nanocomposites demonstrated a discharge energy density of 26.5 J/cm3 with a charge–discharge efficiency of 68% at the optimal composition. Additionally, amorphous nanoparticles have a higher energy barrier than crystalline nanoparticles, which can inhibit the carrier transport more effectively to obtain a lower leakage current. Compared with traditional preparation approaches, the as-proposed method for preparing nanocomposites is simple, and the resulting materials exhibit high performance, which is critical for the application of nanocomposites in capacitors.

Nanoparticle transport within non-Newtonian fluid flow in porous media.

Control over dispersion of nanoparticles in polymer solutions through porous media is important for subsurface applications such as soil remediation and enhanced oil recovery. Dispersion is affected by the spatial heterogeneity of porous media, the non-Newtonian behavior of polymer solutions, and the Brownian motion of nanoparticles. Here, we use the Euler-Lagrangian method to simulate the flow of nanoparticles and inelastic non-Newtonian fluids (described by Meter model) in a range of porous media samples and injection rates. In one case, we use a fine mesh of more than 3 million mesh points to model nanoparticles transport in a sandstone sample. The results show that the velocity distribution of nanoparticles in the porous medium is non-Gaussian, which leads to the non-Fickian behavior of nanoparticles dispersion. Due to pore-space confinement, the long-time mean-square displacement of nanoparticles depends nonlinearly on time. Additionally, the gradient of shear stress in the pore space of the porous medium dictates the transport behavior of nanoparticles in the porous medium. Furthermore, the Brownian motion of nanoparticles increases the dispersion of nanoparticles along the longitudinal and transverse direction.

Experimental investigation of GO-HPAM and SiO2-HPAM composite for cEOR: Rheology, interfacial tension reduction, and wettability alteration

The interest in designing and developing polymeric nanoparticles has grown in recent years. The inclusion of nanoparticles in polymer solutions enhances several properties, such as stability, rheological and shear characteristics, polymer adsorption, wettability alteration, and so on, leading to increased oil recovery. Graphene oxide (GO) nanosheets and Aerosil 300 Silica oxide (SiO2) are the superior nanoparticles because of their robust thermal and mechanical properties. However, a systematic study of the use of GO and SiO2 in hydrolyzed polyacrylamide (HPAM) has not been reported. The present research seeks to improve the stability of nanoparticles, the rheological characteristics of polymeric nanofluids, and to understand the functional interaction of nanoparticles in reducing interfacial tension and altering wettability. The nano polymer composites were characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. The samples were treated with NaCl (30,000 mg/L) and CaCl2 (1000 mg/L) brine solutions at 25 °C and 80 °C. The findings indicated that the addition of GO and SiO2 into HPAM significantly improves rheological properties, reduced IFT, and decreased contact angle. GO performs better than SiO2 in terms of better rheological properties and the reduction of IFT. However, the use of SiO2 in HPAM resulted in a lower contact angle in comparison to GO-HPAM nano-polymeric solution. The synergetic effect of GO and SiO2 with HPAM showed promising results under harsh reservoir conditions and hence can be considered as a good candidate for the application of chemical enhanced oil recovery.

Phenomenological study of the micro- and macroscopic mechanisms during polymer flooding with SiO2 nanoparticles

The incorporation of SiO2 nanoparticles in polymer solutions for oil recovery has generated considerable interest in recent research. Although this approach improves the polymer performance, little evidence is available about the recovery mechanisms that intervene in the polymeric nanofluid suspension or polymer nanofluid. Therefore, the purpose of this study is to describe the microscopic and macroscopic mechanisms involved during oil recovery with polymeric nanofluid flooding. Changes in the rheology and capillary forces (contact angle and interfacial tension) in the polymeric solution due to the incorporation of SiO2 nanoparticles were evaluated. Coreflooding tests were made to evaluate the retention, apparent viscosity, and impact on the relative permeability. Finally, macroscopic oil recovery was measured by displacement tests in a quarter 5-spot system, and microscopic displacement was evaluated by a microfluidic test. Results indicated that the conservation of a favorable mobility ratio in polymeric nanofluids along the porous medium is the phenomenon that most impacts macroscopic oil recovery. This behavior was associated with a reduction in loss retention and the conservation of the apparent viscosity in the porous medium. A direct relationship was observed between the concentration of SiO2 nanoparticles and the viscoelastic behavior of the polymer and the water-wet condition on the surface exposed to the nanofluid. Moreover, residual oil distribution in microscopic displacement revealed that the reduction in residual oil mainly originated from the decrease in the size of the oil clusters. Consequently, the evidence suggests that microscopic oil displacement in polymeric nanofluid is attributed to the reduction in the capillary forces and increases in the viscoelastic nature of the polymer. Results in this work indicated that the additional oil recovery obtained by polymeric nanofluid flooding is generated by the improvement of macroscopic recovery mechanisms and the activation of microscopic mechanisms.

Diffusion Mode Transition between Gaussian and Non-Gaussian of Nanoparticles in Polymer Solutions

The dynamic density functional theory is applied to study the diffusion of nanoparticles in polymer solutions, in which different diffusion modes have been identified by exploiting the density and free energy evolutions. Under the condition of low polymer concentration, diffusion is controlled by particle free motion with a normal Gaussian type. As the concentration increases, the non- Gaussian behavior can be observed when the particle size is comparable to the correlation length of polymer chain. Particles need to penetrate through a cage and overcome an entropic barrier, where the hopping and the model-coupling diffusion coexist. Further increase of polymer concentration can result in complete restriction of the particle by surrounding polymer segments. In this case, the non-Gaussian process fades away, and particle diffusion is controlled by Rouse dynamics of polymer chains with the generalized Gaussian characteristics.

Microwave-Assisted Polyol Synthesis of Pt/Pd and Pt/Rh Bimetallic Nanoparticles in Polymer Solutions Prepared by Batch and Continuous-Flow Processing

Colloidal dispersions of Pt/Pd and Pt/Rh bimetallic nanoparticles have been synthesized by a microwave-assisted polyol method using ethylene glycol and glycerol as a solvent in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). The structure of bimetallic nanoparticles has been investigated by means of high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS) elemental mapping, and extended X-ray absorption fine structure (EXAFS). The effectiveness of the batch and continuous-flow processing was demonstrated for the preparation of various bimetallic nanoparticles under multimode or single-mode microwave irradiation. In the single-mode microwave-assisted continuous-flow processing in contrast to the batch processing, the well-dispersed colloidal bimetallic nanoparticles were successfully produced at the boiling temperature of solvent. EXAFS analysis indicated that, in the case of Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles prepared using both batch and continuous-flow processing, the distribution of different metallic species in a particle tended to be a “cluster-in-cluster” structure.

Scale-dependent rotational diffusion of nanoparticles in polymer solutions.

It is shown that the rotational diffusivity of nanoparticles in polymer solutions spanning the dilute to semi-dilute regimes deviates from the predictions of the Stokes-Einstein (SE) relationship, and that this deviation can be explained by the existence of a polymer depletion layer with the viscosity of the bath solvent. The measurements of the rotational diffusion coefficient of poly(ethylene glycol) (PEG) grafted magnetic nanoparticles in PEG solutions spanning the dilute to semi-dilute regimes and a wide range of polymer molecular weights were obtained from the dynamic magnetic response of the nanoparticles to alternating magnetic fields. Experimental rotational diffusion coefficient values were compared with those predicted by the SE relation using the macroscopic viscosity of the polymer solutions and the hydrodynamic radius of the nanoparticles. Deviations between experimental and SE rotational diffusivity values were observed for nanoparticles in polymer solutions where the radius of gyration of the polymer exceeded the hydrodynamic radius of the particles. A simple model for the rotational hydrodynamic drag on a particle surrounded by a polymer depletion layer was found to describe the experimental rotational diffusivities well, suggesting that the observed phenomenon arises due to the formation of a polymer depletion layer around the nanoparticles.

Nanoparticle diffusion in crowded and confined media.

We identify distinct mechanisms controlling slowing of nanoparticle diffusion through complex media featuring both rigid geometrical confinement and soft mobile crowders. Towards this end, we use confocal microscopy and single particle tracking to probe the diffusion of 400 nm nanoparticles suspended in Newtonian water, in a Newtonian glycerol/water mixture, or in a non-Newtonian polymer solution through a model porous medium, a packed bed of microscale glass beads. The mobility of nanoparticles, as quantified by the long-time diffusion coefficient extracted from the particle mean-squared displacement, slows as the average pore size of the packed bed media decreases for both Newtonian and non-Newtonian solutions. The distribution of particle displacements is non-Gaussian, consistent with the spatial heterogeneity of the geometrical confinement imposed by the packed bed. The slowing of nanoparticle mobility in all solutions follows the predictions of models that describe hydrodynamic interactions with the packed bed. In non-Newtonian solutions, depletion interactions due to the polymers near the glass beads result in temporary adsorption of particles onto the bead surface, as indicated by a stretched-exponential distribution of residence times. Our results therefore suggest that the confined diffusive dynamics of nanoparticles in polymer solutions is controlled by two competing mechanisms: hydrodynamic interactions between particles and spatial obstacles, which dictate the long-time slowing of diffusion, and depletion interactions between particles and confining walls due to the macromolecules, which control transient adsorption and hence alter the statistics of the short-time motion.

Reentrant Stabilization of Grafted Nanoparticles in Polymer Solutions.

Polymer chains grafted onto nanoparticles may facilitate the dispersion of such particles in a polymer solution. We explore the optimal strategy for stabilizing polymer-grafted nanoparticles using self-consistent field theory and experiments. The best results are obtained for relatively low grafting densities and for chain lengths of the brush polymer NB larger than that of the freely floating polymers Nf. When Nf > NB, one finds a compatibilization gap and re-entrant stabilization: At both very low and very high polymer concentrations particles disperse in the polymer solution, while at intermediate concentrations the particles lose their colloidal stability. At low grafting densities the underlying surface is in contact with the solvent. Particles covered by a bidisperse brush can combine a low grafting outer region with full coverage of the surface by a densely grafted inner layer. Using classical colloid-chemical stabilization criteria the region in the phase diagram for which the particles are expected to mix with a concentrated polymer solution opens up. Now, also upon an increase in the length of the freely dispersed polymers, a re-entrant colloid-chemical stabilization is found for particles on the nanometer length scale: At both short and long polymer chains in solution the particles will not aggregate, whereas at intermediate lengths the colloidal stability is marginal. This multi re-entrant behavior is found from numerical self-consistent field calculations, and these predictions are consistent with corresponding experiments.

Diffusion of nanoparticles in semidilute polymer solutions: A mode-coupling theory study.

We have proposed a theoretical formalism to study the long-time diffusion behavior of nanoparticles in polymer solutions by using mode-coupling theory (MCT). The non-hydrodynamic part Dmicro of the total diffusion coefficient D is calculated in the MCT framework where the polymer dynamic scattering function Γpp(k, t) in the solution plays an important role. By introducing an approximate summation form for Γpp(k, t), where both limits of short and long length scales are properly accounted for, we can compute Dmicro straightforwardly and investigate explicitly how D depends on the volume fraction ϕ of the polymer solution, the nanoparticle size R, the degree of polymerization N, as well as the entanglement effects. For illustration, we adopt our theoretical approach to analyze the diffusion of gold nanoparticles in semidilute poly(ethylene glycol) (PEG)-water solutions which has been studied in detail experimentally. We find that our theoretical results show very good quantitative agreements with the experimental data in many aspects, such as the strong dependence on ϕ, the large deviation from Stokes-Einstein relation particularly for small particles, as well as the effects of the PEG molecular weight. Such good agreements clearly demonstrate the validity of our MCT framework, which may serve as a good starting point to study many more complex dynamical behaviors associated with polymer solutions.

Effect of confinement on polymer-induced depletion interactions between nanoparticles

Using a numerical implementation of polymer mean-field theory, we probe the effects of a structureless wall on the insertion free energies and the depletion interactions between nanoparticles in polymer solutions. Our results indicate that the insertion free energies and the polymer-induced interactions become mitigated in the presence of a wall. The range of influence of the walls is shown to correspond to the correlation length of the polymer solution. Surprisingly, our results demonstrate that even for particle sizes comparable to the correlation length of the polymer solution, the polymer depletion density profiles near the wall (in the absence of particles) can be used as a means to quantitatively predict the influence of the wall on both the insertion free energies and the depletion interactions.

Effect of solvent quality on the dispersibility of polymer-grafted spherical nanoparticles in polymer solutions

In this work, lattice-based self consistent field theory is used to study the structural properties of individual polymer-grafted spherical nanopartices and particle-particle interactions in polymer melts and solutions under variable solvent conditions. Our study has focused on the depth of the minimum in the potential of mean force between the two brush-coated nanoparticles, if such a minimum occurs, and we have also addressed the corresponding radial density profiles of free and grafted chains around a single nanoparticle, in an attempt to clarify the extent of correlation between the depth of the minimum, W(min), and the parameter δ characterizing the interpenetration between the profiles of free and grafted chains. Although one cannot establish a simple one-to-one correspondence between W(min) and δ, we do find common trends, in particular, if the solvent conditions for free and grafted chains differ: varying the volume fraction of the free chains, δ typically exhibits a broad minimum, corresponding to a region where the magnitude of W(min) exceeds thermal energy k(B)T, leading to particle aggregation.

Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions

Direct observation of time-resolved emission spectra (TRESs) of cadmium sulfide nanoparticles in polymer solutions was carried out with picosecond resolution using a streak camera. The TRESs were found to undergo a pronounced time-dependent Stokes shift, eventually coinciding with the steady-state photoluminescence spectra within an approximately 40 ns delay time. Moreover, approximately 90% of the shift was complete within the first 1 ns after excitation, in contrast to the fact that overall photoluminescence involves very long time constants of 10-100 ns. The observed Stokes shift dynamics was very similar in CdS nanoparticles stabilized in two very different types of polymer solutions. Thus the solvent and/or polymeric stabilizer appeared to have a minimal effect on the shift. We propose that the relaxation proceeds through an internal mechanism involving the fast decay of high-energy traps into relatively slow-decaying low-energy traps. Time-dependent photoluminescence anisotropy experiments also revealed an approximately 1 ns decay component appearing only in the higher-energy end of the photoluminescence spectrum. Because this time constant is too short to represent rotational diffusion of the nanometer-sized particles, it was associated with the rapid relaxation of the high-energy trap states.

Formation of the bimodal ensemble of silver nanoparticles in polymer solutions

Formation of the bimodal ensemble of silver nanoparticles in polymer solutions

Formation of the bimodal ensemble of silver nanoparticles in polymer solutions

Silver nanoparticles synthesized in polymer solutions are studied by spectroscopy (in near ultraviolet, visible, and infrared spectral regions) and high-resolution transmission electron microscopy. It is established that the ensemble with bimodal particle size distribution is formed from initially polydisperse nanoparticle ensemble in methylhydroxy ethyl cellulose solution. In contrast to the classical scheme of the ripening of colloidal dispersions, the number of small particles increases with time in the studied system; moreover, particles with a size of about 2 nm turned out to be stable. The large particles grow with time and their concentration drops.

Formation of the bimodal ensemble of silver nanoparticles in polymer solutions

Formation of the bimodal ensemble of silver nanoparticles in polymer solutions

Structuring and sedimentation stability of titanium phosphate nanoparticles in polymer solutions

The influence of dispersed phase nature on the sedimentation stability as well as coagulation structure of titanium phosphate (TP) nanoparticles in polymer suspensions has been investigated. Two systems are considered: (i) TP suspension in toluene/ethyl cellosolve mixed solution of ephoxy resin E-40 and (ii) multicomponent system, ferric oxide, talc and TP suspension in toluene/ethyl cellosolve solution of E-40, as a model of the practical varnish–paint systems. For the case of a polar solvent, a unique phenomenon, extremal dependence of the strength of model systems on the concentration TP, is detected and increase of sedimentation stability of TP organic suspensions with time of contact of TP with a solvent is observed. Data of FTIR-spectroscopy show that phenomena detected result from the formation adsorption shell possessing high structural viscosity on TP nanopaticles.